|John Broskie's Guide to Tube Circuit Analysis & Design|
20 December 2002
As expected, any article that even mentions a tube phono preamp garners a large response. Vinyl just will not die. Beyond the sonics, what I dearly miss is the packaging: 12 inches by 12 inches was a perfectly sized canvas for liner notes and, most importantly, artwork. I knew many people who framed record covers and I have bought many a record because of the artwork on its cover alone; no one in the history of the CD has bought a CD for the same reason. Consider this: we have lost an art form without any fuss from the consuming public; yet when graffiti is painted over, there is an outcry from art professors.
When I first read of the CD's creation as a playback medium, I prayed that the LP's form factor would be retained and that we would be sold single-sided Laser Disks. Because this 12-inch disk is twice as large, it can hold four times the content of a CD. Imagine the possibilities: longer recording times and higher sampling rates and word lengths. No such luck. In fact, the need to make a walkman-compatible medium was a decisive consideration and Philips originally only wanted 14 bits of resolution, which Sony be thanked, did not happen. The audio DVD restores the content potential, but still saddles us with the small cases. What would have proved interesting was if the music labels sold two disks, one 12 inch high-quality digital disk and one low-res (14 bit and 30kHz sampling) mini-disk (it might have fit in the larger one's hole), both packaged together, which would have made for smaller walkmans and car playback decks, while still retaining a high-end home playback medium. Dream on, John.
The ZEN amplifier is an intriguing design but with many limitations. First, let’s pull way back and think about what is going on in the amplifier: one MOSFET is doing all the work of amplifying the 1-volt input signal to 16 volts and driving 2-amps of current into the speaker and providing a low output impedance; all at once! What fuels this miracle? It cannot be the wimpy 1-volt input signal. No, it must the MOSFET’s transconductance which gives us voltage and current gain. Thus, the greater the transconductance, the higher the gain and the lower the output impedance. For example, a Gm of 4A per volt would gives us, with an input signal of 1V peak, a gain of 32 into an 8-ohm load.
Zen amplifier topology
The output impedance, however, would be next to infinity, as the MOSFET has little if any “rp.” If we apply a feedback loop across the amplifier, the gain might drop to 16 and the output impedance would then be 4 ohms, for example, or a gain of 1 and an output impedance of 0.25 ohms, that is if all the output were returned to the input. Notice the tradeoff: we can have high gain, but high output impedance or a low output impedance, but low gain. We cannot, however, have both high gain and low output impedance. Well, what’s the big deal here? Just use a higher transconductance MOSFET or place a few MOSFETs in parallel to boost the transconductance.
Two problems: high transconductance MOSFETs are usually meant to be used at high currents, currents much higher than even our Class-A SE amplifier will draw. Unfortunately, the MOSFET’s transconductance is not a perfect constant, as it decreases at low current, which means that an 8A/V MOSFET might only have 4A/V of transconductance at 2A. Second, the higher the transconductance, the higher the input capacitance. Placing many MOSFETs in parallel will also multiply the input capacitance. Have you ever wondered why the ZEN amp uses such a low valued input series resistor (2K) for a voltage driven device? Why not use a 47k input resistor, concidering that the device is voltage driven and presents a high input impedance? Unfortunately, the high input capacitance limits the value of this resistor, if a wide bandwidth is expected.
Now, in general, triodes have a much lower input capacitance than MOSFETs do, but if we divide the Gm by the input capacitance, I am not sure which device would win. One advantage the triode enjoys over the MOSFET is that it comes with a built-in feedback mechanism in the form of its plate resistance. So, for example, if 10 6BQ5s were placed parallel, the effect rp would be a low 200 ohms, which when divided by the square of the output winding ratio, becomes a small 2 ohms. Not bad. Wait a minute, this value seems to imply that the triode is beating the MOSFET in every regard, but 10 6BQ5s only have a combined Gm of only 0.11A/V. Two tricks were involved here. The first is the world’s simplest SE amplifier would see an input signal closer to 20V peak, effectively increasing the Gm by twentyfold. Second, it takes twenty times more current to drive a capacitance to 20V rather than 1V, which means that a robust line stage would be needed, just as in the MOSFET example.
Now, you can see the difficulty of using just one output device to generate gain and deliver a low output impedance. Adding one extra gain stage effectively amplifies the Gm of the output device by its gain. Now that extra gain stage might be found in your line stage for free or it might have to included in the amplifier itself.
Tube CAD does the hard math for you.
This program covers 13 types of tube circuits, each one divided into four variations: 52 circuits in all. Tube CAD calculates the noteworthy results, such as gain, phase, output impedance, low frequency cutoff, PSRR, bias voltage, plate and load resistor heat dissipations. Which tube gives the most gain? Tube CAD's scenario comparison feature shows which tube wins.
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